In modern supply circuits 106, as shown in FIG. 1, the supply current of the motor 105 is generated by semiconductor switches 103 switched at high frequency. The current I(t) actually generated typically has a frequency of a few tens of kHz, whilst the fundamental frequency If of the supply current, having a sinusoidal shape overall represented by the envelope of the current I(t), is relatively low, for example around a few hundreds of Hz, as illustrated in FIG. 2a. The technological advances achieved in the design of power semiconductors enable the switches 103 to have extremely high switching speeds, the potential variation speed dv/dt being for example around 10 to 20 kV/μs, as illustrated in FIG. 2b. In order to contain the electrical radiation caused by such potential variation speeds, use is made of coaxial cables 104 for supplying the motor 105. Since these cables are highly capacitive, and having regard to the dv/dt applied, stray high-frequency (HF) currents are generated in the form of oscillations damped at each switching. The amplitude and frequency of these currents are of the same order of magnitude whatever the driving power. This is because they depend practically only on the characteristics of the coaxial cables used and the amplitude of the dv/dt applied. The amplitude of these currents can reach several tens of amperes and their frequencies range from 100 kHz to 1 MHz.
The current sensors 101 are generally placed on lines 102a, 102b, 102c supplying the motor 105. Although these HF currents do not have to be measured, they nevertheless pass through the current sensors. In drives 106 of small and medium power, the amplitude of these stray currents may be much higher than those of the currents necessary for controlling the motor. FIG. 2c shows, on an oscilloscope screen, the voltage U(t) and the high-frequency current I(t) due to the switchings and to the capacitive loads on one phase of a 5.5 kW motor supplied by a supply circuit switched at 16 kHz. In this example, the amplitude of the first and second half-wave I1,I2 is approximately 20 A and 8 A respectively. In practice an amplitude at the first half-wave I1 and I2 of 20 A and 30 A peak is normal.
The inventors have realised that this causes two main problems. The first is an increase in the thermal current passing through the sensor, which can be resolved by sizing the sensor as a function of the sum of the rms currents which pass through it. Another problem is very great heating of the magnetic circuit due to losses by hysteresis and losses by eddy currents.
It is necessary to emphasise that these problems are not found in sensors of the “closed loop” type since, to within any compensation errors, the primary ampere-turns (At) are compensated for by the secondary ampere-turns.
It should be noted that the heating of the magnetic circuit will be all the higher, and therefore difficult, the smaller the size of the sensor. This is due to the sizing constraints with small open-loop current sensors. This is because, for reasons of measuring precision, it is not appropriate to design a sensor below a minimum level of 40 ampere-turns. This means that a sensor of nominal size 10 A will be designed with 4 primary turns whilst a 40 A nominal sensor can be designed with simply 1 primary turn. Thus, in the first case, the amplitude of the HF currents and the resulting magnetic induction will be multiplied by 4 compared with the second case and consequently the heating due to the losses by hysteresis and the dynamic losses will be 16 times greater, as can be deduced from the following relationship:Losses(W)≈f2B2d2/φwhere d is the thickness of the magnetic plates, B is the magnetic induction, f is the frequency of the induction and therefore of the stray current HF and φ is the resistivity of the ferromagnetic alloy constituting the magnetic circuit of the sensor.
Tests show that temperatures from 200° C. to 300° C., or even more, would be reached with open-loop sensors of small size and traditional construction if they were used as they stood in the applications described above.
In practice, this type of sensor can be used only if there is disposed on its primary connections a related circuit which diverts the HF currents; however, this circuit has the drawback of destroying the dynamic performance of the sensor and thus restricts the efficiency of the drive. For these reasons, this type of sensor is not, up to the present time, used in drives for high-performance motors; it is replaced by a sensor of the more expensive “closed loop” type.